1. Field of the Invention
The present invention relates to a composite ceramic candle filter for removing particulates from a hot gas stream, and a method for making said filter.
2. Description of Related Art
Ceramic filters have been tested in processes such as coal gasification and coal combustion to remove particulates from hot flue gases to protect downstream equipment from corrosion and erosion and to comply with EPA NSPS (New Source Performance Standards) regulations. Ceramic filters in a tubular (candle) form, with one end closed and the other end open have been shown to remove the particulates efficiently. The hot gas to be filtered typically flows from the outside to the inside of the filter, with particulate-free gas exiting from the open end. The candle geometry is also suited for removal of the filtered cake by backpulsing with compressed gases.
Ceramic hot-gas candle filters must withstand exposure to chemically corrosive gas streams at temperatures in excess of 800 degrees C. In addition, they are subjected to significant thermal stresses during backpulse cleaning which can cause catastrophic failure of the ceramic candle filter element.
Ceramic hot-gas candle filters known in the art are generally fabricated from either porous monolithic materials or porous ceramic fiber-containing composite materials. Monolithic ceramic candle filters are either weak or can fail catastrophically in use. Composite filters are less susceptible to catastrophic failure and generally have improved strength, toughness, and thermal shock resistance versus monolithic ceramic filters.
Candle filters may have relatively uniform porosity throughout the filter or they may comprise a porous support with a thin layer, or membrane, of fine porosity on the outer surface of the support. The membrane layer is typically applied to the filter using a variety of methods such as coating from a dispersion containing finer grains than those used in the support for smaller membrane pore sizes, bonding randomly arranged chopped ceramic fibers to the support using colloidal (or sol) materials, or forming a ceramic matrix by chemical vapor infiltration.
Materials used to fabricate ceramic hot-gas filters generally include oxides such as aluminosilicates, glass, and alumina, and non-oxides such as silicon carbide and silicon nitride. Oxide-based ceramic filters have adequate resistance to flue gas atmospheres and fly-ash for the design life of the filters however they generally have low thermal shock resistance. Non-oxide ceramics generally have good thermal shock resistance, however they are susceptible to oxidation in the corrosive environment to which they are subjected which results in a degradation of mechanical properties.
The disadvantages of ceramic candle filters known in the art include failure, often catastrophic, due to thermally induced stresses caused by backpulse cleaning, chemical degradation caused by species present in the hot gases being filtered, delamination of the membrane layer, incomplete removal of the filter cake upon backpulsing, and high cost. They also tend to be heavy, requiring expensive support structures to hold an array of the candles in the filter unit.
The present invention provides a strong, lightweight ceramic hot-gas candle filter which has a greater than 99.5% particulate collection efficiency, thus meeting EPA NSPS regulations. The filter of the present invention comprises a filament wound support having a membrane layer applied to the outer surface thereof. The support comprises a reticulated tube of yarns comprising ceramic oxide fibers which are at least partially coated with a porous refractory oxide matrix. The membrane layer comprises an ordered arrangement of continuous filament ceramic oxide yarns which are also coated with a porous refractory oxide matrix. The membrane layer is firmly adherent to the support and therefore does not suffer from delamination problems. The porosities of the support and membrane are controlled such that the support functions as a bulk filter and the membrane layer functions as a surface filter. Failure of the filter is generally not catastrophic since if the membrane is damaged, the support quickly blinds at the location of the damage due to its bulk filtration properties, thus preventing release of particulates and protecting downstream process equipment such as gas turbines or sorbent beds. The filter of the present invention is resistant to chemical degradation due to the oxide compositions used, and at the same time provides excellent thermal shock resistance which is not generally typical of oxide materials. The smoothness of the membrane surface results in efficient removal of the filter cake during backpulse cleaning. In addition to the above-mentioned advantages, the filter of the current invention is potentially low cost relative to most of the commercially available candle filters.